The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium. The FCC process is carried out by contacting the starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. During the cracking reaction, coke will be deposited on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, if it is desired to increase the conversion rate an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected. Since the catalyst temperature in the regeneration zone is usually held at a relatively constant temperature, significantly higher than the reaction zone temperature, any increase in catalyst flux from the relatively hot regeneration zone to the reaction zone affects an increase in the reaction zone temperature.
Reductions in the availability of relatively light FCC feeds has made it necessary at times to process feeds in FCC units that have higher molecular weights and an increased tendency to lay down coke deposits on the catalyst in the reaction zone and carry over light combustible products from the reaction zone to the regeneration zone. Increased production of coke and the combustion of relatively low molecular weight cracking products generates more heat in the regenerator as the coke is burned from the surface of the catalyst. The resulting higher catalyst temperatures can adversely influence the catalyst to oil ratio in the reactor riser and result in catalyst deactivation. A number of regeneration techniques are practiced to minimize or remove the heat released by the combustion of large amounts of coke in the regenerator. These methods include the direct removal of heat by the use of catalyst that use a cooling fluid to cool the catalyst by indirect heat exchange, and operation of the regeneration of the regeneration zone to limit the combustion of CO to CO.sub.2. However these methods have typically required multiple regeneration stages or additional contacting vessels and neither of these methods eliminates the loss of relatively low molecular weight products in the regeneration zone.
The hydrocarbon product of the FCC reaction is recovered in vapor form and transferred to product recovery facilities. These facilities normally comprise a main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked products which usually include bottom materials, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams.
As the development of FCC units has advanced, temperatures within the reaction zone were gradually raised. It is now commonplace to employ temperatures of about 525.degree. C.(975.degree. F.). At higher temperatures, there is generally a loss of gasoline components as these materials crack to lighter components by both catalytic and strictly thermal mechanisms. At 525.degree. C., it is typical to lose 1% on the potential gasoline yield due to gasoline components thermally cracking into lighter hydrocarbon gases. As temperatures increase, to say 1025.degree. F.(550.degree. C.), most feedstocks lose up to 6% or more of the gasoline yield due to thermal cracking of gasoline components.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor and are transported upward by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, if present, upon contact with the hot catalyst. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds. A number of riser designs use a lift gas as a further means of providing a uniform catalyst flow. Lift gas is used to accelerate catalyst in a first section of the riser before introduction of the feed and thereby reduces the turbulence which can vary the contact time between the catalyst and hydrocarbons.
The benefits of using lift gas to pre-accelerate and condition regenerated catalyst in a riser type conversion zone are well known. Lift gas typically has a low concentration of heavy hydrocarbons, i.e. hydrocarbons having a molecular weight of C.sub.3 or greater are avoided. In particular, highly reactive type species such as C.sub.3 plus olefins are unsuitable for lift gas. Thus, lift gas streams comprising steam and light hydrocarbons are generally used.
The most readily available source for lift gas is from the gas concentration section of an FCC unit's product separation facilities. Taking lift gas from the gas concentration section places additional demands upon these facilities. Since the lift gas passes from the riser through the reactor and back to the gas concentration section, it is essentially a recycle stream. The additional throughput associated with the lift gas reduces the overall gas processing capabilities of the gas concentration facilities. Therefore, obtaining lift gas from the gas concentration section increases the cost of providing and maintaining the gas concentration section and where the gas concentration capacity limits the throughput through the FCC unit, the use of lift gas may be precluded or may reduce the amount of fresh feed processed in the FCC.
Further improvements in reduction of product losses and the control of regeneration temperatures have been achieved by providing multiple stages of catalyst stripping and raising the temperature at which the catalyst particles are stripped of products and other combustible compounds. Both of these methods will increase the amount of low molecular weight products that are stripped from the catalyst and will reduce the quantity of combustible material in the regenerator. A variety of arrangements are known for providing multiple stages of stripping and heating the spent catalyst to raise the temperature of the stripping zone. With increasing frequency it is being proposed to raise the temperature of the stripping zone by mixing the spent catalyst with hot regenerated catalyst from the regeneration zone.